Amplifier circuit audio circuit and electronic device

ABSTRACT

An amplifier circuit which, includes a first input for receiving a first input signal and a second input for receiving a second input signal. A first amplifier section is connected to the first input and the second input is present. The first amplifier section can combine the first signal and the second signal into a first combined signal, the first amplifier being connected to a first output, for outputting the first combined signal. A second amplifier section is connected to the first input and the second input, for combining the first signal and the second signal into a second combined signal, the second amplifier being connected to a second output, for outputting the second combined signal. A common mode section is connected to the first input and the second input, for generating a common mode signal based on a combination of the first signal and the second signal and outputting the common mode signal at a common output connected to the common mode amplifier. The first combined signal, the second combined signal and the common mode signal satisfy the condition that combining the first combined signal and the common mode signal results in a signal with an amplitude proportional to the amplitude of the first input signal and combining the second combined signal and the common mode signal results in a signal with an amplitude proportional to the amplitude of the second input signal.

FIELD OF THE INVENTION

This invention relates to an amplifier circuit, to an audio circuit, andto an electronic device.

BACKGROUND OF THE INVENTION

Amplifier circuits are generally known in the art and used, for example,to provide amplified signals to the loudspeakers of a headset. Forexample, amplifier circuits are known which can be connected to aheadset via a three or four pole jacket and plug configuration. Betweenthe signal outputs of the amplifier circuit and the headset, large valuecoupling capacitors are typically present. The large value couplingcapacitors are expensive and occupy a relatively large area.

Amplifier circuits in which the need for the large value couplingcapacitors is obviated are known. Referring to FIG. 1 of theaccompanying drawings, an amplifier circuit 10 as is known from theprior art is shown. The amplifier circuit 10 is connected to a headset20 which comprises two loudspeakers 210,220 each with respectivenegative and positive contacts 211,212;221,222. As shown the negativecontacts 212,222 are both connected to the same common ground contact202 and the positive contacts 211,221 are connected to separate signalcontacts 200,201. The amplifier circuit 10 has stereo channel inputs100,101 at which a left channel signal and a right channel signal canrespectively be received. At shown, the left channel input 100 isconnected to an input 121 of a left channel amplifier 120. The output122 of the left channel amplifier 120 is connected to a left channeloutput contact 110 of the amplifier. The right channel input 101 isconnected to an input 131 of a right channel amplifier 130. The output132 of the right channel amplifier 130 is connected to a right channeloutput contact 111 of the amplifier circuit 10. As shown in FIGS. 2A and2C, at the amplifier outputs 122,132, amplified channel signals areoutputted which have an amplitude proportional to the amplitude of theleft channel signal and the right channel signal respectively.

The left channel output contact 110 is connected to one signal contact200 of the headset 20 and the right channel output contact 111 isconnected to the other signal contact 201. The amplifier circuit 10further includes an operational amplifier 140 which is connected to aground contact 112. The ground contact 112 is connected to the commonground contact 202 of the headset 20. The operational amplifier 140provides a “virtual” ground voltage equal to the common mode of the leftchannel signal and the right channel signal. As illustrated in FIG. 2B,at the ground contact a constant voltage is outputted which is equal tothe amplifier gain voltage VAG (short for virtual analog ground) set atthe positive input of the operational amplifier. Typically, theamplifier gain voltage VAG is set to half the supply voltage Vcc.

However, although the shown prior art circuit allows the use of aconventional headset with a three pole plug without requiring couplingcapacitors between the amplifier circuit 10 and the headset 20, adisadvantage of the prior art circuit is that the circuit 10 requires anaccurate determination of the common mode in order to set amplifier gainvoltage VAG. Furthermore, the circuit 10 consumes a relatively largeamount of power and has a relatively large footprint because of theoperational amplifier 140.

SUMMARY OF THE INVENTION

The present invention provides an amplifier circuit, an audio circuit,and an electronic device and as described in the accompanying claims.

Specific embodiments of the invention are set forth in the dependentclaims.

These and other aspects of the invention will be apparent from andelucidated with reference to the embodiments described hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

Further details, aspects and embodiments of the invention will bedescribed, by way of example only, with reference to the drawings.

FIG. 1 shows a block diagram of a configuration of an amplifier circuitand a headset as known from the prior art.

FIGS. 2A-2C schematically show graphs of the voltage as a function oftime at the channel contacts of the amplifier circuit shown in FIG. 1.

FIG. 3 shows a block diagram of an example of a configuration of a firstexample of an embodiment of an amplifier circuit and a headset.

FIGS. 4A-4C schematically show examples of graphs of the voltage as afunction of time at the contacts of the example of an embodiment of anamplifier circuit shown in FIG. 3.

FIGS. 5A and 5B schematically show examples of graphs of the differencevoltage as a function of time between the channel contacts and thecommon ground contact of the example of an embodiment of an amplifiercircuit shown in FIG. 3.

FIG. 6 shows a block diagram of an example of a configuration of asecond example of an embodiment of an amplifier circuit and a headset.

FIG. 7 shows a block diagram of an example of an embodiment of anamplifier section.

FIG. 8 shows a block diagram of an example of an example of anembodiment of an audio circuit.

FIG. 9 shows a block diagram of an example of an embodiment of anelectronic device.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring FIG. 3, an example of an embodiment of an amplifier circuit 30is shown therein. The amplifier circuit 30 may, as shown, include afirst input 300 for receiving first input signal and a second input 301for receiving a second signal. As shown in FIG. 3, the amplifier circuit30 may include a first amplifier section 32, a second amplifier section33, as well as a common mode section 34.

In the following, the amplifier circuit 30 will be elucidated furtherusing the example of a stereo audio signal which comprises a leftchannel signal which is to be outputted at a loudspeaker at a first sideand a right channel signal which is to be outputted at a loudspeaker ata second side. The first input signal will be referred to as the leftchannel signal, the second input signal as the right channel signal, thefirst input will be referred to as the left channel input 300 and thesecond input will be referred to as the right channel input 301. Thefirst amplifier section 32 will be referred to as the left channelamplifier section 32 and the second amplifier section will be referredto as the right channel amplifier section 33. However, it will beapparent that the amplifier circuit 30 may be used to amplify othertypes of signals.

The left channel amplifier section 32 may be connected to the leftchannel input 300 and the right channel input 301. The left channelamplifier section 32 may generate a first combined signal based on acombination of the left channel signal L and the right channel signal Rand output the first combined signal at a first output 310 connected tothe left channel amplifier 32. The right channel amplifier section 33may be connected to the left channel input 300 and the right channelinput 301.

The right channel amplifier section 33 may generate a second combinedsignal based on a combination of the left channel signal L and the rightchannel signal R and output the second combined signal at a secondoutput 311 connected to the right channel amplifier 33.

The common mode section 34 may be connected to the left channel inputand the right channel input. The common mode section 34 may generate acommon mode signal based on a combination of the first signal and thesecond signal and output the common mode signal at a common output 312connected to the common mode amplifier 34.

The first combined signal and the common mode signal may satisfy thecondition that combining the first combined signal and the common modesignal results in an amplified left channel signal with an amplitudeproportional to the amplitude of the left channel input signal. Thesecond combined signal and the common mode signal may satisfy thecondition that combining the second combined signal and the common modesignal results in an amplified right channel signal with an amplitudeproportional to the amplitude of the right channel input signal.

Thus, the (amplified) left and right channel signals may be obtainedfrom the outputs 310-312 of the amplifier circuit 10. Thereby, the needto provide a virtual ground and to provide an operational amplifierconnected to the common mode output is obviated. Accordingly, the powerconsumption and size of the amplifier circuit 10 may be reduced.Furthermore, a good isolation between the right channel and left channelcan be obtained. Also, as shown in FIG. 3, the amplifier circuit 30 maybe used with existing audio systems, e.g. headphones, for example thosein which the loudspeakers are connected with a three wire cable and athree pole plug.

The first combined signal s₁ and the common mode signal s_(common) mayfor example satisfy the mathematical relationship:ƒ₁(s ₁(t),s _(common)(t))=ƒ(s ₁(L(t),R(t)), s_(common)(L(t),R(t)))=a·L(t),  (1)

in which equation f₁(s₁,s_(common)) represents the combining function,L(t) the left channel signal, R(t) the right channel signal, arepresents an amplification factor in the range of 0 to ∞ and trepresents time.

The second combined signal s₂ and the common mode signal s_(common) mayfor example satisfy the mathematical relationship:ƒ₂(s ₂(t), s _(common)(t))=ƒ₂(s ₂(L(t),R(t)), s_(common)(L(t),R(t)))=a·R(t),  (2)

in which equation f₂(s₂,s_(common)) represents the combining functionand R(t) the right channel signal. In equations (1) and (2), theamplification factor a may be constant or vary in time or as a functionof another parameter, such as the amplitude of the respective channelsignal. The amplification factor or gain a may for example be constantfor channel signals with an amplitude in a certain range in which casethe amplifier circuit operates as a linear amplifier for channel signalswith an amplitude in that range.

The first combined signal s₁ and the common mode signal s_(common) mayfor example satisfy the condition that superimposing the first combinedsignal s₁ on the common mode signal s_(common) or vice versa results inan amplified left channel signal with an amplitude proportional to theamplitude of the left channel signal L. The second combined signal s₂and the common mode signal s_(common) may for example satisfy thecondition that superimposing the second combined signal s₂ on the commonmode signal s_(common) or vice versa results in an amplified rightchannel signal with an amplitude proportional to the amplitude of theright channel signal R.

For example, the first combined signal s₁ and the common mode signals_(common) may for example satisfy the mathematical relationship:s ₁(L(t),R(t))−s _(common)(L(t),R(t))=a·L(t),  (3)

The second combined signal s₂ and the common mode signal s_(common) mayfor example satisfy the mathematical relationship:s ₂(L(t),R(t))−s _(common)(L(t),R(t))=a·Rt),  (4)

For instance, the first combined signal s₁ may be obtained by asuperposition of the left channel input signal and the right channelinput signal to obtain a combined signal. As shown in FIG. 3, forinstance, one or more of the sections 32-34 may include a combining unit35 connected to the left channel input 300 and to the right channelinput 301. The combining unit may perform a superposition of the leftchannel input signal L and the right channel input signal R to obtain acombined signal.

The superposition in the left channel amplifier section 32 may forexample be an operation as may be described by the mathematicalrelationship:S ₁(t)=L(t)−R(t),  (5)

As shown in FIG. 3, for instance the combining unit 35 may subtract theright channel signal R from the left channel signal L and be connectedwith a positive input to the left channel input 300 with a negativeinput to the right channel input 301.

In the second amplifier section 33 the second combined signal s₂ may forexample be obtained by a superposition of the left channel input signaland the right channel input signal to obtain a combined signal, forexample as may be described by the mathematical relationship:s ₂(t)=−L(t)+R(t),  (6)

As shown in FIG. 3, for instance the combining unit 35 in the rightchannel amplifier section 33 may subtract the left channel signal L fromthe right channel signal R and be connected with a negative input to theleft channel input 300 and with a positive input to the right channelinput 301.

For instance, in the common mode section 34 the common mode signals_(common) may be obtained by a superposition of the left channel inputsignal and the right channel input signal to obtain a combined signal,for example as may be described by the mathematical relationship:s _(common)(t)=−L(t)−R(t),  (7)

As shown in FIG. 3, for instance the combining unit 35 in the commonmode section 34 may subtract the left channel signal L from an invertedright channel signal R and be connected with a negative input to theleft channel input 300 and with a negative input to the right channelinput 301.

Referring to FIGS. 4 and 5, in FIG. 4A the output of the left channelamplifier section is shown, supposing that the left channel input signalL and the right channel input signal R correspond to those shown inFIGS. 2A and 2C. In FIG. 4B, the output of the common mode section isshown. In FIG. 4C the output of the right channel amplifier section isshown.

As shown in FIG. 5A and B, an amplified left or right channel signal canbe obtained by combining the output of the left channel amplifier 32 (orthe right channel amplifier 33) and the output of the common modesection 34, for example by superimposing the first or second combinedsignal s₁,s₂ and the common mode signal s_(common). As shown in FIG. 3,for example, the first output 310 and the common mode output 312, aswell as the second output 311 and the common mode output 312, may beconnected to opposite, e.g. positive and negative contacts 411,412 of aload to be driven. The load driven by the amplifier circuit may be anysuitable type of load and for example be one or more transducers whichcan convert the first and second amplified channel signals intorespective acoustic signals, e.g. loudspeakers 410,420 of e,g. a headset40. Thus, the load is driven by the difference between the firstcombined signal s₁ and the common mode signal e.g. bys₁−S_(common)=L−R−(−L−R)=2L (that is by the amplified left channelsignal, which in this example is obtained by a linear superposition ofthe first combined signal s₁ on the common mode signal).

The sections 32-34 may be implemented in any manner suitable for thespecific implementation. As shown in FIG. 3, two or more of the leftchannel amplifier section 32, the right channel amplifier section 33 andthe common mode section 34 may be similar. In the example of FIG. 3 allsections have the same design. Thereby design time may be reduced andmatching of the outputs 310-312 may be relatively simple.

The combining unit 35 may for instance be implemented as separate unitconnected to an amplifier 34, e.g. an adder which is connected with thesuitable inputs to an inverter, and thus receives an inverted signal,e.g. an inverted left channel signal −L and/or an inverted right channelsignal −R. However, as illustrated in FIG. 6, the combining unit 35 maybe integrated in the amplifier 34, and for example be implemented in asumming amplifier. In the example of FIG. 6, resistors R_(L), R_(R) areconnected to the left and right channel input respectively, and may forexample receive the left or right channel signal or an inverted left orright channel signal. An amplifier resistor R_(A) connects the output ofthe amplifier A to the input of the amplifier. The output V_(A) of theamplifier A is thus equal to V_(A)=−R_(A)(V_(L)·R_(L)+V_(R)·R_(R)) withV_(L) and V_(R) representing the voltage of the left channel signal andthe right channel signal respectively.

As illustrated in FIG. 3, each of the sections 32-34 may include arespective amplifier 36 which is positioned, in a signal processingdirection, downstream of the combining unit 35. The amplifier 36 may beany type of amplifier suitable for the specific implementation, such asa Class A or Class A/B amplifier or be a switched amplifier, such as aclass D amplifier.

In the example of FIG. 3, in each section 32-34 the combining unit 35 isconnected with a first input 350 to the left channel input 300 andconnected with a second input 351 to the right channel input 301. Theoutput 352 of the combiner unit 35 is connected to an amplifier input360 of the amplifier 36. The amplifier output 361 of the amplifier 36 isconnected to a respective one of the outputs 310-312 of the amplifiercircuit 30.

However, the amplifier 36 may be positioned, in the signal processingdirection, upstream of the combining unit 35. As illustrated in FIG. 7,for example, a left channel amplifier 36 may be present between the leftchannel input 300 and the amplifier sections 32-34 and a right channelamplifier 36 may be present between the right channel input 301 and theamplifier sections 32-34. In this example, each amplifier 36 isconnected with the amplifier input 360 to a respective channel input300,301. The amplifier output 361 is connected to a respective input350,351 of the combining unit of the sections 32-34. The output 352 ofthe combining unit 35 is connected to a respective output 310-312 of theamplifier circuit 30.

The amplifier circuit 30 may be implemented as an audio amplifier, thatis an amplifier which amplifies signals composed primarily offrequencies between 20 hertz to 20,000 Hertz, such as low-power audiosignals to a level suitable for driving loudspeakers and is suitable asthe final stage in a typical audio playback chain connected to theloudspeakers. Although other values may be used, it is found thatsuitable values for an amplifier used for driving loudspeakers may forexample be a gain of between 0 and 40 dB, such as 39 dB or less. It isfound that suitable values for an amplifier used for driving headphonesare, for example, at least 6 dB and/or not more than 12 dB.

Referring to FIG. 8, the amplifier circuit 30 may for example form partof an audio circuit for a set 40 of two or more loudspeakers 410,420.The audio circuit may include a signal processing unit 50 having a firstsignal output 510 for outputting a first processed audio signal and asecond signal output 511 for outputting a second process audio signaland the amplifier circuit 30. The amplifier circuit 30 may be connectedwith the left channel input 300 to the first signal output 510 of thesignal processing unit 50 and with the right channel input 301 to thesecond signal output 511. The signal processing unit 50 may for exampleperform, in operation, functions like pre-amplification, equalization,tone control, mixing/effects and may be connected with a signalprocessing input 500 to sources of audio signals such as one or more of:radio receiver, optical disk players, non-volatile memory with audiodata stored thereon or other suitable source of audio signals.

As illustrated in FIG. 9, the amplifier circuit may include a firstconnector for connecting the first output and the common mode output ofthe amplifier circuit to a first load and a second connector forconnecting the second output and the common mode output to a secondload. The amplifier circuit may for example include a multi-pole jackconnector having at least three poles of which a first pair of polesforms the first connector and a second pair of poles forms the secondconnector. In the example of FIG. 9, for instance, the outputs 310-312are connected to respective poles 81-83 of a three pole socket 80. Asshown, the loudspeakers of the headset may be connectable to the audiocircuit with a plug 90 which is compatible with the socket 80.

As shown in FIG. 9, the input of the signal processor 50 may beconnected to a source of audio signals, in this example to anon-volatile memory 60 on which data representing audio signals can bestored, for example in a compressed form.

In the foregoing specification, the invention has been described withreference to specific examples of embodiments of the invention. It will,however, be evident that various modifications and changes may be madetherein without departing from the broader spirit and scope of theinvention as set forth in the appended claims. For example, theconnections may be an type of connection suitable to transfer signalsfrom or to the respective nodes, units or devices, for example viaintermediate devices. Accordingly, unless implied or stated otherwisethe connections may for example be direct connections or indirectconnections.

Also, for example, although in the present document reference has beenmade to headsets and headset interface circuits being interconnectedusing two or more wires, these wires will generally be bundled in asingle cable.

Also, the invention is not limited to physical devices or unitsimplemented in non-programmable hardware but can also be applied inprogrammable devices or units able to perform the desired devicefunctions by operating in accordance with suitable program code.Furthermore, the devices may be physically distributed over a number ofapparatuses, while functionally operating as a single device. Forexample, the amplifier circuit 30 may be implemented as a number ofseparate integrated circuits (e.g. which each include one of thesections 32-34) connected to form the amplifier circuit

Also, devices functionally forming separate devices may be integrated ina single physical device. For example, the amplified circuit 30 and thesignal processor 50 may be implemented as a single integrated circuit,for example as a single die or as two or more dies packaged in a singleintegrated circuit package.

However, other modifications, variations and alternatives are alsopossible. The specifications and drawings are, accordingly, to beregarded in an illustrative rather than in a restrictive sense.

In the claims, any reference signs placed between parentheses shall notbe construed as limiting the claim. The word ‘comprising’ does notexclude the presence of other elements or steps then those listed in aclaim. Furthermore, the words ‘a’ and ‘an’ shall not be construed aslimited to ‘only one’, but instead are used to mean ‘at least one’, anddo not exclude a plurality. The mere fact that certain measures arerecited in mutually different claims does not indicate that acombination of these measures cannot be used to advantage.

The invention claimed is:
 1. An amplifier circuit, comprising: a firstinput for receiving a first input signal; a first amplifier connected tosaid first input for outputting a first amplified signal based upon thefirst input signal; a second input for receiving a second input signal;a second amplifier connected to said second input for outputting asecond amplified signal based upon the second input signal; a firstcombiner section connected to said first amplifier and said secondamplifier, for combining said first amplified signal and said secondamplified signal into a first combined signal, said first combinersection being connected to a first output, for outputting said firstcombined signal; a second combiner section connected to said firstamplifier and said second amplifier, for combining said first amplifiedsignal and said second amplified signal into a second combined signal,said second combiner section being connected to a second output, foroutputting said second combined signal; and a common mode sectionconnected to said first amplifier and said second amplifier, forgenerating a common mode signal based on a combination of said firstamplified signal and said second amplified signal and outputting saidcommon mode signal at a common output connected to said common modesection, wherein the first combined signal, the second combined signaland the common mode signal satisfy the condition that combining thefirst combined signal and the common mode signal results in a thirdamplified signal with an amplitude proportional to the amplitude of thefirst input signal and combining the second combined signal and thecommon mode signal results in a fourth amplified signal with anamplitude proportional to the amplitude of the second input signal. 2.An amplifier circuit as claimed in claim 1, wherein the first combinedsignal, the second combined signal and the common mode signal satisfythe condition that superimposing the first combined signal on the commonmode signal or vice versa results in a signal with an amplitudeproportional to the amplitude of the first input signal andsuperimposing the second combined signal on the common mode signal orvice versa results in a signal with an amplitude proportional to theamplitude of the second input signal.
 3. An amplifier circuit as claimedin claim 1, wherein at least one of said sections comprises: a combiningsection connected to said first amplifier and to said second amplifier,for performing a superposition of said first input signal and saidsecond input signal to obtain a combined signal.
 4. An amplifier circuitas claimed in claim 1, wherein: said first output and said common outputare connectable to a first transducer for converting said thirdamplified signal into a first acoustic signal and said second output andsaid common output are connectable to a second transducer for convertingsaid fourth amplified signal into a second acoustic signal.
 5. Anamplifier circuit as claimed in claim 1, wherein said first input signaland said second input signal are stereo audio signals.
 6. An amplifiercircuit as claimed in claim 1, wherein at least one of said amplifiersincludes a switched amplifier, wherein a switched amplifier includes aclass D amplifier.
 7. An amplifier circuit as claimed in claim 1,wherein at least two of said first combiner section, said secondcombiner section and said common mode section are similar.
 8. Anamplifier circuit as claimed in claim 1, including a first connector forconnecting said first output and said common mode output of saidamplifier circuit to a first load and a second connector for connectingsaid second output and said common mode output to a second load.
 9. Anamplifier circuit as claimed in claim 8, wherein said first load andsaid second load are loudspeakers.
 10. An amplifier circuit as claimedin claim 8, including a multi-pole jack connector having at least threepoles of which a first pair of poles forms said first connector and asecond pair of poles forms said second connector.
 11. An audio circuitfor a set of at least two loudspeakers, comprising: a signal processingunit having an input for receiving at least one audio signal, a firstsignal output for outputting a first processed audio signal and a secondsignal output for outputting a second processed audio signal; and anamplifier circuit as claimed in claim 1, with said first input of saidamplifier circuit connected to said first signal output of said signalprocessing unit and with said second input of said amplifier circuitconnected to said second signal output of said signal processing unit.12. An electronic device including: a source of audio signals and anaudio circuit as claimed in claim
 11. 13. An amplifier circuit asclaimed in claim 2, wherein at least one of said sections comprises: acombining section connected to said first amplifier and to said secondamplifier, for performing a superposition of said first input signal andsaid second input signal to obtain a combined signal.
 14. An amplifiercircuit as claimed in claim 2, wherein: said first output and saidcommon output are connectable to a first transducer for converting saidthird amplified signal into a first acoustic signal and said secondoutput and said common output are connectable to a second transducer forconverting said fourth amplified signal into a second acoustic signal.15. An amplifier circuit as claimed in claim 3, wherein: said firstoutput and said common output are connectable to a first transducer forconverting said third amplified signal into a first acoustic signal andsaid second output and said common output are connectable to a secondtransducer for converting said fourth amplified signal into a secondacoustic signal.
 16. An amplifier circuit as claimed in claim 2, whereinsaid first input signal and said second input signal are stereo audiosignals.
 17. An amplifier circuit as claimed in claim 3, wherein saidfirst input signal and said second input signal are stereo audiosignals.
 18. An amplifier circuit as claimed in claim 4, wherein saidfirst input signal and said second input signal are stereo audiosignals.
 19. An amplifier circuit as claimed in claim 2, wherein atleast one of said amplifiers includes a switched amplifier.
 20. Anamplifier circuit as claimed in claim 3, wherein at least one of saidamplifiers includes a switched amplifier, wherein a switched a includesa class D amplifier.